Cell hybrid culture lines producing monoclonal antibodies: a)fusion of myeloma antibody producing cells, in order to obtain clonal B cell hybrid culture lines which secrete large quantities of monoclonal antibodies (Mabs) reactive with lymphocyte and other lymphoid cell surface determinants. b) Use of these Mabs for genetic and functional studies of various lymphocyte subpopulations. Lymphocyte surface determinants and subpopulations: a) Use of Mabs and conventional antisera to identify and characterize lymphocyte cell surface determinants, especially those which mark functional lymphocyte subpopulations or progressive stages of differentiation of individual subpopulations. c) Functionally characterize lymphocyte subpopulations carrying different surface markers. c) Genetically map loci controlling lymphocyte surface determinants.